1. Field of the Invention
The present invention relates generally to superplastic forming of a workpiece; and, more specifically to a forming tool and method used to form a workpiece.
2. Description of Related Art
Superplastic forming (SPF) takes advantage of a material's superplasticity or ability to be strained past its rupture point under certain elevated temperature conditions and strain rates. Superplasticity in metals is defined by very high tensile elongations, ranging from two hundred to several thousand percent. SPF is a process that can be used to produce parts that takes advantage of the high elongation behavior of certain superplastic materials.
SPF typically includes the steps of heating a sheet of material to a point in which superplastic deformation is possible, clamping the material within a sealed die and then using gas pressure to force the material to stretch and take the shape of a forming surface located in the die cavity. Controlling the gas pressure during the forming process controls the deformation rate of the material and maintains superplasticity at the elevated temperature.
Superplastic forming process can lead to part thinning and strain localization in certain areas, such as corners of the mold cavity. Typically, the workpiece stretches or thins in a uniform manner in the open area of the die cavity. Once the workpiece engages the mold surface of the die cavity, deformation or thinning in the contact area is restricted resulting in a greater amount of workpiece stretching or thinning in the last area to contact the die surface. To increase control of the thinning problem and correspondingly control thickness distribution the workpiece can be pre-stretched. That is, the workpiece is pre-stretched or pre-thinned in certain areas depending upon the ultimate configuration of the die cavity. Once the workpiece is successfully pre-stretched, pressure is applied to form the workpiece by pressing it against the mold surface of the forming die.
One process for pre-stretching a workpiece entails blow forming the workpiece away from the molding surface to create a pre-form bubble. After sufficiently stretching the workpiece, reverse pressure then forces the workpiece into the die cavity and against the mold surface. In order to perform such a process, the forming tool must be designed to have a sealed pressure system on both sides of the workpiece and include a pre-form cavity located in the forming tool opposite the molding surface.